This invention relates generally to a nonwoven fabric or web which is formed from spunbond fibers of a thermoplastic resin, and laminates using such a web as a component. The fabric is used as a filter, particularly for industrial applications where the fabric is in the form of a sock.
Thermoplastic resins have been extruded to form fibers, fabrics and webs for a number of years. The most common thermoplastics for this application are polyolefins, particularly polypropylene. Other materials such as polyesters, polyetheresters, polyamides and polyurethanes are also used to form nonwoven spunbond fabrics.
Nonwoven fabrics or webs are useful for a wide variety of applications such as diapers, feminine hygiene products, towels, recreational or protective fabrics and as geotextiles and filter media. The nonwoven webs used in these applications may be simply spunbond fabrics but are often in the form of nonwoven fabric laminates like spunbond/spunbond (SS) laminates or spunbond/meltblown/spunbond (SMS) laminates.
As filter media, some of the desired characteristics of nonwoven fabrics are that they be permeable to the fluid being filtered yet have a high filtration efficiency. Permeability to the fluid being filtered is quite important as low permeability could result in a high pressure drop across the filter requiring a higher, and hence more costly, energy input into the filtered fluid and shortening filter life. In a dust sock application, a fan is generally used to force air through the sock. A sock having low permeability increases the back pressure on the fan and so increases the energy input required to move the same amount of fluid and shortens the fan's life.
High filtration efficiency is, of course, the main purpose for a filter and great efficiency and ability to maintain the efficiency at an acceptable level are key to filter performance.
It is an object of this invention to provide a spunbond polyolefin nonwoven fabric or web for use as a filter medium which has a high permeability and high filtration efficiency. It is a further object of this invention to provide a dust sock filter made from the filter medium.